PHP-FPM service fails to start on Plesk: An another FPM instance seems to already listen on php-fpm.sock

Symptoms

1. PHP-FPM service fails to start on Plesk Onyx 17.8.
2. Error in /var/log/plesk-phpXX-fpm/error.log:

   ERROR: An another FPM instance seems to already listen on /var/www/vhosts/system/subdomain.example.com/php-fpm.sock
   ERROR: An another FPM instance seems to already listen on /var/www/vhosts/system/subdomain.example.com/php-fpm.sock
   ERROR: FPM initialization failed
   ERROR: FPM initialization failed

3. Subdomain was created via XML API.
4. Non-system PHP handler is assigned to the subdomain mentioned in the error message. But configuration file from system PHP handler presents:

   # find /etc /opt -name subdomain.example.com.conf
   /etc/httpd/conf/plesk.conf.d/vhosts/subdomain.example.com.conf
   /etc/nginx/plesk.conf.d/vhosts/subdomain.example.com.conf
   /etc/php-fpm.d/subdomain.example.com.conf

Cause

Incorrect PHP configuration file was generated during subdomain creation via XML API.

Plesk bug PPPM-10022 which will be fixed in future updates. 

Resolution

Apply one of the workarounds below:

1. Log in to Plesk server via SSH
2. Remove the excessive configuration file:

   # rm -f  /etc/php-fpm.d/subdomain.example.com.conf

OR

1. Log in to Plesk
2. Go to Domains > subdomain.example.com > PHP Settings 
3. Switch to FPM by OS vendor and back to the desired PHP version